The effect of high fiber fraction on some mechanical properties of unidirectional glass fiber-reinforced composite

Objectives

This study was designed to evaluate the effect of an increase of fiber-density on some mechanical properties of higher volume fiber-reinforced composite (FRC).

Methods

Five groups of FRC with increased fiber-density were fabricated and two additional groups were prepared by adding silanated barium-silicate glass fillers (0.7 μm) to the FRC. The unidirectional E-glass fiber rovings were impregnated with light-polymerizable bisGMA-TEGDMA (50-50%) resin. The fibers were pulled through a cylindrical mold with an opening diameter of 4.2 mm, light cured for 40 s and post-cured at elevated temperature. The cylindrical specimens (n = 12) were conditioned at room temperature for 2 days before testing with the three-point bending test (Lloyd Instruments Ltd.) adapted to ISO 10477. Fiber-density was analyzed by combustion and gravimetric analyzes.

Results

ANOVA analysis revealed that by increasing the vol.% fraction of E-glass fibers from 51.7% to 61.7% there was a change of 27% (p < 0.05) in the modulus of elasticity, 34% (p < 0.05) in the toughness, and 15% (p < 0.05) in the load bearing capacity, while there was only 8% (p < 0.05) increase in the flexural strength although it was statistically insignificant. The addition of particulate fillers did not improve the mechanical properties.

Significance

This study showed that the properties of FRC could be improved by increasing fibervolume fraction. Modulus of elasticity, toughness, and load bearing capacity seem to follow the law of ratio of quantity of fibers and volume of the polymer matrix more precisely than flexural strength when high fiber-density is used.     

Orthodontics in general practice. 4. Eruption guidance appliances in orthodontics

Eruption guidance appliances are recommended for early orthodontic treatment or prevention of malocclusions. The treatment effect of eruption guidance appliances and functional appliances is similar. In addition to dentoalveolar and skeletal effects, eruption guidance appliances would also have myofunctional effects for treating open mouth behaviour and swallowing problems. However, there is no solid evidence for the myofunctional effect claimed. The position of erution guidance appliances in the orthodontic treatment arsenal is limited: early treatment of Angle Class II malocclusion in 2 phases has no advantage over a 2 phase treatment. When eruption guidance is needed, preference is given to an individually produced appliance.     

In vitro study of mean loads and modes of failure of all-ceramic crowns cemented with light-cured or dual-cured luting cement, after 1 and 30 d of storage

This study compared the mean loads and modes of failure of teeth restored with all-ceramic crowns (ACCs) cemented with dual-cured (RelyX ARC; 3M ESPE) or light-cured (RelyX Veneer; 3M ESPE) luting cements. Clinically, there are advantages of light-cured cements over the recommended dual-cured cements, namely increased working time, improved handling, colour stability, and a homogenous mix. Forty, sound, extracted, human, premolar teeth underwent a standardized preparation for ACCs. IPS Empress (Ivoclar-Vivadent) crowns of standard dimensions were fabricated and 20 were cemented with each cement. The crowns were stored for 1 or 30 d in water and subjected to a compressive load to failure at 0.017 mm s⁻¹. There were no significant differences in loads at failure, between each cement group, at each storage period, and there were no significant differences in loads at failure, for each cement, at 1 and 30 d of storage. There were also no significant differences in modes of failure between each cement group. Before recommending light-cured cement as an alternative to dual-cured cement for the cementation of all-ceramic crowns, further research is required to establish the depth of ceramic at which light-cured lutes fail to polymerize completely.     

Five-year study of Class II restorations in permanent teeth using amalgam, glass polyalkenoate (ionomer) cermet and resin-based composite materials

The aim of this study was to compare the clinical performance of an amalgam, a glass polyalkenoate (ionomer) cermet material and a resin-based composite material used in small Class II cavities in permanent teeth. All restorations were inserted under rubber dam. They were examined yearly for 3 years. One clinician continued the study up to 5 years. The clinical examination focused on two criteria: clinically acceptable and failure. In addition, impressions were taken of the prepared cavities immediately before restoration and at each clinical examination using an elastomeric material. The study comprised 274 Class II restorations (88 amalgams, 95 cermets and 91 resin composites) placed in 142 adolescent patients. One hundred and sixty-seven restorations were in molar and 107 in premolar teeth. Patient dropout after 5 years resulted in the loss of 161 restorations, evenly distributed for restorative material and type of tooth involved. Four amalgam restorations, 22 glass ionomer cermet and nine resin composite restorations failed. The glass ionomer cermet and amalgam restorations failed primarily due to bulk fractures, while the resin composite restorations failed due to secondary caries and bulk fractures.     

Frozen autogenous mandible as an immediate replacement graft.

A study in animals was designed to examine the feasibility of grafting autogenous frozen mandible. Two groups of animals were used. In the first group, periosteum was elevated from the bone before resection. In the second group, the periosteum was removed with the resected specimen. Osteogenesis was shown by clinical, radiographic, fluorescence, and histologic studies in both groups of animals. The same technique was performed in nine humans with carcinomas that involved the mandible. The results were encouraging. We believe that immediate or delayed reconstruction with autogenous frozen mandible in a patient undergoing ablative surgery of the head and neck with involvement of the bone may offer the patient, as well as the surgeon, another acceptable treatment modality.     

Observation of orthopedic force distribution produced by maxillary orthodontic appliances

A three-dimensional anatomic model was duplicated from a human skull, using different birefringent materials to simulate the various craniofacial structures. Individual bones of the midface were fabricated separately and then articulated in their correct sutural relation. One removable and four fixed maxillary appliances were used. The fixed appliances included the Haas, Minne-expander, Hyrax, and quad helix devices. The removable appliance incorporated an expansion screw in a full acrylic palate with appropriate retentive clasps. After the insertion of each appliance, intraoral forces were produced by incremental activation. The model was examined and photographed in the field of a transmission polariscope. Each appliance used produced a different range of load-activation characteristics. This was reflected by the differences in the stresses transmitted through the bones of the craniofacial complex and the effect on the various sutures. Stresses produced by the fixed appliances were concentrated in the anterior region of the palate, progressing posteriorly toward the palatine bone. The Haas, Minne-expander, and Hyrax appliances produced stresses that radiated superiorly along the perpendicular plates of the palatine bone to deeper anatomic structures, such as the lacrimal, nasal, and malar bones, as well as the pterygoid plates of the sphenoid. Similar stress characteristics were seen with the removable appliance. However, increased activation decreased retention of the appliance, thereby lessening the stress. The quad helix appliance proved to be the least effective orthopedic device. Although the effects of palate separation were seen with increased activation, this appliance primarily affected the posterior teeth.